1. Technical Field
The invention relates to inkjet printers. More particularly, the invention relates to an inkjet printer that has a dot alignment vision system.
2. Description of the Background Art
An image to be printed in an ink jet printer is finally a map of dots with x and y coordinates for each dot. If all of the dots are in the correct position, the expected quality is achieved. The ideal dot has a circular shape and a determinate size. There are various factors that affect the ideal dot.
The drop of ink fired by an inkjet lands in the media and forms an irregular shaped dot that is close to having the shape of a circle, but that is not perfectly circular. Because the jetpack is moving when it fires, the final shape of the dot consists of a main dot and some smaller satellite dots. Changing the direction of the moving jetpack changes this pattern, such that the satellite dots are now on the other side of the main dot. Also, the speed at which the jetpack moves affects the final shape of the dot.
Most printers have the option of unidirectional or bidirectional printing. For productivity reasons, the bidirectional mode is the preferred mode. In this mode, the printer must be adjusted such that the dots printed from right to left are kept aligned to the dots printed from left to right. That is, the x coordinate of any dot should be correct no matter the printing direction. This is the bi-directional adjustment.
When an array of jetpacks, each having multiple nozzles, is printing, the media is still and the firing nozzles form lines horizontally. Then, the media advances and a new pass is made and the printed lines interlace until the complete set of the image dots are printed. When this advance distance is correct, the y coordinate of each dot is in place. This is the step adjustment.
The final shape and size of a dot also depends in the distance between the jet nozzles and the printed media and in the amount and temperature of the drop of ink fired.
When a nozzle is disabled, i.e. it does not fire ink, a blank space is left in the map of dots that form the image affecting the final quality.
Inkjet printers' quality is achieved by positioning the dots forming an image precisely. The higher the printed resolution, the smaller the dots are. Today, in the Very Grand Format segment of the printers industry, the resolutions can be over a thousand Dots Per Inch (DPI) and the tolerances can be smaller than a thousand of an inch.
Traditionally, a person performs printer adjustments by first analyzing a printed pattern with the naked eye or using an eye loop. Because these adjustments are within few thousands or even fractions of a thousand of an inch, even using a microscope, a more precise and automated method is needed to eliminate subjective quality determination. While a person typically must analyze test patterns and determine adjustment values for most very grand format printers, some printers use sensors that help to analyze printed patterns.
One problem with having a person adjust an inkjet very grand format printer, even using visual aids to analyze the adjustment patters, is the subjective quality determination and the limitation of the human eye to determine small (=<0.001″) adjustment values with precision.
The sensors used today in some printers are fixed image systems that use a grid to determine if a printed pattern aligns with a mask (see Cobbs; U.S. Pat. No. 5,600,350), and that pattern is only printed in one section of the printing area, therefore not taking into account imperfections of the platen or carriage moving system. This last statement has been addressed by others and they create a table using an external measurement system to create a table and/or a special encoder strip.
It would be advantageous to provide a more precise and automated method to eliminate subjective quality determination when aligning inkjet printers.